1. Field of the Invention
The present invention relates to a technique which is effective when used in a printing device, and further, a printing device (printer) with a printhead being in pressure contact with a platen roller and with a printing paper sheet sandwiched between the printhead and the platen roller for conducting printing. More particularly, the present invention relates to a thermal printing device (thermal printer) which can conduct satisfactory printing on printing paper sheets of different thicknesses.
2. Description of the Related Art
In a thermal printer which conducts printing with roll thermal paper (hereinafter referred to as roll paper) inserted in between a printhead having a heater element (hereinafter referred to as a thermal head) and a platen roller, because the thermal head is in pressure contact with the platen roller, when the roll paper is loaded, it is necessary to release the thermal head from the platen roller and to insert a leading edge of new roll paper thereinbetween.
For example, Patent Document 1 (JP 2003-251837 A) and Patent Document 2 (JP 2006-116714 A) disclose a conventional technique for making it easier to insert roll paper in between a thermal head and a platen roller when the roll paper is loaded. In these patents, a cam is disposed at the back of the thermal head. The cam is adapted to be rotated to, against repulsion of a spring, press the thermal head against the platen roller. The cam is also adapted to be rotated in an opposite direction to move, using the repulsion of the spring, the thermal head away from the platen roller to create clearance therebetween into which roll paper is to be inserted.
Further, as a technique for making it easier to set roll paper between a thermal head and a platen roller, there is a clam shell printer disclosed in, for example, Patent Document 3 (JP 2000-318260 A) in which the platen roller is detachable.
In order to conduct correct printing by a thermal printer, it is necessary to press a thermal head against a platen roller and to position a heater element of the thermal head in a range where a surface of the platen roller made of rubber becomes flat (which is called a nip). Various kinds of thermal paper for printing are offered in the market, and it is required to conduct high quality printing on any kind of thermal paper.
However, if the thickness of the thermal paper varies and still the thermal head is pressed against the platen roller with the same pressure, there is a possibility that the width of the nip varies and the quality of the printing is lowered. More specifically, as the thickness of the thermal paper becomes larger and the thermal paper becomes sturdy, the width of the nip becomes smaller, and thus, it is desirable that the pressing force of the thermal head be made to be larger as the thickness of the thermal paper becomes larger. However, in a conventional thermal printer, because a function of changing the pressing force of the thermal head is not provided, there is an inconvenience that the quality of the printing varies depending on the paper thickness.
A problem of the structures disclosed in Patent Documents 1 and 2 that paper is difficult to insert is solved in the clam shell as disclosed in Patent Document 3. FIG. 6 illustrates a conventional structure of a unit used in a clam shell printer. In this unit, hemispherical bearing concave portions 12a and 12b with which a shaft of a platen roller 11 can be engaged is formed at tips of arms 12A and 12B of a holder 12 for holding a head support plate 15 having a thermal head 14. The shaft of the platen roller 11 is adapted to be detachably engaged with the bearing concave portions 12a and 12b. A clam shell printer with such a unit also does not have a pressure changing mechanism as described in the present application, and, pressure necessary for thick paper is applied such that all recommended paper sheets can be used without the need for adjustment. However, the high pressure means that torque transferred by a motor becomes larger, and power consumption of and heat generated by the motor are bad. Also, because load applied to a gear which transfers power becomes heavier, the durability of the gear is decreased. Therefore, it is revealed that to accommodate both thin paper and thick paper with only one model forces a customer who only uses thin paper to put up with a lower specification due to a useless function, and, on the other hand, to set an additional model which accommodates thick paper is not preferable from the viewpoint of inventory control.
Accordingly, the present inventors studied making adjustable the pressing force of the thermal head according to the paper thickness. In the inventions described in Patent Document 1 and Patent Document 2 described above, a cam is disposed at the back of the thermal head, and the pressing force is adapted to be applied to the thermal head using a repulsion of a leaf spring brought in contact with the cam or of a compression spring inserted between a plate brought in contact with the cam and the thermal head. By using such a cam, it is also possible to change the pressing force of the thermal head.
However, a thermal printer to which the inventions described in Patent Document 1 and Patent Document 2 are applied is relatively large, and thus, there is room to dispose the cam at the back of the thermal head. On the other hand, in a small thermal printer which the present inventors regard as the target, there is not enough room at the back of the thermal head and it is difficult to dispose the cam. Therefore, if a cam is used to change the pressing force of the thermal head, there is a problem that the apparatus becomes large.
Further, because, in the printers described in Patent Document 1 and Patent Document 2, the position of the platen roller is not movable with respect to the position of a member for rotatably holding the platen roller, it is necessary to keep enough distance between the platen roller and the thermal head when roll paper is loaded, and thus, the cam is used to move the thermal head a relatively large distance. Therefore, when the thermal head is moved by means other than the cam (for example, a feed screw), there are inconveniences that it takes much time to move the thermal head and the number of parts is increased.
Further, in the printers described in Patent Document 1 and Patent Document 2, it is also possible that the cam for making it easier to load roll paper is adapted to be used to change the pressing force of the thermal head, but it is revealed that, if such a structure is adopted, because it is necessary to adjust the position of the cam every time a user loads roll paper such that optimum pressure can be obtained according to the thickness of the paper to be used, not only the setting is burdensome but also inappropriate pressure may be set by accident.